Improved orthopaedic procedures have made joint replacement common practice. In total hip arthroplasty, both the femoral or ball aspect of the joint as well as the acetabular or socket portion are replaced with prosthetic implants. Several patent and non-patent references exist in this field.
Orientation of the femoral stem is universally considered as less complex. Orientation of the acetabular component is far more complicated, and techniques designed to improve it are far less developed. Some of the present techniques are inadequate due to the complexity. The literature has shown a lack of accuracy even among experienced surgeons when estimating intraoperative orientation of the acetabular cup with standard techniques.
Some techniques developed for computer assisted surgery are promising but present limitations of their own and it may be some time before patients can benefit. Its world wide availability is limited due to high costs and special training required. There are several reasons for the lack of progress in this area. The orientation of the acetabular cup requires an adequate orientation of the pelvis in three planes. Its anatomy may be altered by disease, even severely altered in cases of revision arthroplasty. Present mechanical techniques use the floor level and the longitudinal axis of the operating table as a reference. Some devices use bubble levels, see e.g. U.S. Pat. No. 6,302,890, U.S. Pat. No. 5,700,268 or U.S. Pat. No. 5,122,145, the content of which are incorporated by reference herein, but they only provide an indication with respect to one direction (mono-axis). The initial position of the pelvis in three planes (Yaw, Roll and Pitch) is ignored and therefore can lead to errors. The initial position is also known to change significantly during the surgical intervention. The second problem with standard orientation devices is the assumption that the orientation of the pelvis in terms of flexion/extension is always the same with respect to the axis of the table. Additionally, standard mechanical orientation devices rely on visual references and therefore lack precision. In Minimally invasive surgery (MIS), small surgical incisions make it even more difficult to obtain adequate orientation of the components.
Many surgeons orient the acetabular component through subjective “feeling”, a technique that has also been shown in the literature to be highly inaccurate and imprecise even in the most experienced hands. Studies in saw bone models comparing free hand techniques and standard mechanical orientation devices with computer assisted navigation have clearly shown the lack of accuracy and precision of both techniques. Lack of optimal orientation of both components acetabular cup and shaft as a couple, has been shown to result in limited range of motion due to impingement which leads to dislocation, early wear and loosening of the components.
The situation has lead to the use of imperfect techniques associated with prosthetic acetabular implantation, often involving the use of manually operated instrumentation which rely only on the surgeons hand and eye coordination to install the implant and occasionally relying on visual markers.
Studies have confirmed the importance of precise and matching positioning of both the acetabular and the femoral component. There is increasing awareness that improved reproducibility in component orientation fosters improved range of motion (before impingement of the implant to the cup prosthesis), implant longevity, reduced wear of the acetabular component and lower rates of dislocation.
Clinical epidemiology studies have extensively studied the sources of random errors (lack of precision) and systematic errors (lack of accuracy) for diagnostic or measuring tools such as the Gravity Assisted Navigation System in clinical practice. They have been divided into three aspects:                Observer variability        Subject variability (Differences among patients)        Instrument variability        
There is therefore a need for a simple to use and inexpensive instrumentation and techniques that can be applied to any type of hip prosthesis in either lateral or dorsal decubitus.